1. Field of the Invention
This invention relates in general to a planetary traction drive transmission, and, more particularly, to a planetary traction drive transmission with at least one flexible roller having an adaptive self-loading mechanism.
2. Description of Related Art
Traction drives use frictional force to transmit torque and power. Because the power is transmitted between two smooth surfaces, often through a thin layer of lubricant, a traction drive possesses unique characteristics that are not readily attainable by gear drives. These characteristics include quietness, high-efficiency, high rotational accuracy, and zero-backlash.
Generating adequate normal force at the contact is essential for traction drives. Various loading mechanisms have been proposed. These mechanisms have lead to a host of designs. A common practice is to use tapered surfaces along the axial direction. By moving these surfaces axially, a radial displacement and thus normal force are generated. Examples of such designs are disclosed in U.S Pat. Nos. 3,475,993 and 3,375,739.
Since the envelopes of the tapered surfaces in most designs do not necessarily converge to a common point, this results in a so-called spin motion at contacting surfaces. The spin motion not only offsets the high-efficiency otherwise provided by the traction drive, but also causes component wear and high break away torque.
Recently, a design of zero-spin planetary traction drive has been proposed by Ai as disclosed in the U.S. Pat. No. 6,095,940. This design employs the on-apex concept similar to that of tapered roller bearings. Two rows of planetary rollers are used to balance the internal axial force on the planetary rollers. Although this design offers torque actuated loading mechanism and greater torque capability, it is somewhat complex in construction.
The cylindrical planetary traction drive is also able to achieve zero-spin motion. However, generating sufficient normal force at the contacts has been a challenge. Designs proposed in the past have offered various means to pre-load the drive either by mechanically deforming the outer rings or by thermal assembling the drive. The pre-load generated by such means, in general, can not be adjusted during operation. For partial load application, traction drives are unnecessarily overloaded. This has negative impacts on transmission efficiency and service life.
Perhaps the simplest means to generate torque responsive load is using eccentric planetary drives as was disclosed by Dieterich U.S. Pat. No. 1,093,922 in 1914. Over the years, various improvements have been proposed. See for example, U.S. Pat. Nos. 3,945,270, 4,481,842, 4,555,963, and foreign patent numbers JP10-311398, EP 0,856,462 A2. They all have multiple planetary rigid rollers, and each planetary roller requires a supporting shaft.
While friction actuated loading mechanisms have been disclosed in prior art where one of the planetary rollers was entrained by friction force into the wedged-space created by eccentric raceways, the loading roller was rigid and thus the entrainment angle was kept constant. This was based on the notion that the maximum traction coefficient was constant during the operation. The entrainment angle was determined based on a specified friction coefficient, which was assumed to be not a function of the contact load. Such a loading mechanism was either over conservative or inadequate when the maximum available traction coefficient changes with contact load.
For eccentric planetary drives, the planetary rollers have different sizes. In most of the prior art designs, this has led to different contact stresses at the contact between the sun roller and planets.
In some designs proposed in prior art, the drive had more than one loading roller that is moveable. This could cause a change in eccentricity between input and out put shafts as the loading roller adjust their positions.
Therefore, it is desirable to provide a simple and improved design that allows for an adaptive, torque responsive loading mechanism where the entrainment angle changes as load varies to match the variation in the maximum available traction coefficient, a design that offers balanced contact stresses for all planet rollers and that reduces or eliminates the change in eccentricity as loading roller adjusting its position.